Simplify the following expression: $2\sqrt{99}$.
Explanation: First, try to factor a perfect square out of the radical. $= 2\sqrt{99}$ $= 2\sqrt{9 \cdot 11}$ Separate the radical and simplify. $= 2\sqrt{9} \cdot \sqrt{11}$ $= 2 \cdot 3 \cdot \sqrt{11}$ $= 6\sqrt{11}$